Selective removal of hydrogen sulfide over a zinc oxide and silica absorbing composition

ABSTRACT

Hydrogen sulfide is removed from a fluid stream containing hydrogen sulfide by contacting the fluid stream under suitable absorbing conditions with an absorbing composition consisting essentially of zinc oxide and silica. Additionally, the absorbing composition may contain binders and may be promoted, preferably with nickel oxide.

This is a division of continuation application Ser. No. 875,005, filedApr. 24, 1992, now U.S. Pat. No. 5,248,489, which is a continuation ofapplication Ser. No. 07/363,030, filed Jun. 7, 1989, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an improved process for selectively removinghydrogen sulfide from gaseous streams.

The removal of sulfur from fluid streams can be desirable or necessaryfor a variety of reasons. If the fluid stream is to be released as awaste stream, removal of sulfur from the fluid stream can be necessaryto meet the sulfur emission requirements set by various air pollutioncontrol authorities. Such requirements are generally in the range ofabout 10 ppm to 500 ppm of sulfur in the fluid stream. If the fluidstream is to be burned as a fuel, removal of sulfur from the fluidstream can be necessary to prevent environmental pollution. If the fluidstream is to be processed, removal of the sulfur is often necessary toprevent the poisoning of sulfur sensitive catalysts or to satisfy otherprocess requirements.

A variety of methods employing regenerable, solid contact materials areknown for removing sulfur from a fluid stream when the sulfur is presentas hydrogen sulfide. For example, U.S. Pat. No. 4,088,736 discloses acomposition comprising zinc oxide, alumina, and a Group IIA metal whichis an effective absorbing composition for hydrogen sulfide and whichpossesses the property of being regenerable to the original absorbingcomposition state in the presence of oxygen when fully sulfided.

Although the absorbing compositions employed in such methods mayeffectively absorb hydrogen sulfide from a fluid stream containinghydrogen sulfide, it has been found that many of these absorbingcompositions effectively oxidize significant amounts of hydrogen sulfideto sulfur dioxide. The resulting sulfur dioxide is not absorbed by theabsorbing compositions and, thus, passes unabsorbed through the contactmaterial. In view of the fact that environmental concerns are focused onthe total amount of sulfur contained in an effluent stream, and not justthe amount of hydrogen sulfide, passing sulfur dioxide through thecontact material and out to the environment is not acceptable undercurrent environmental standards.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide an improvedprocess for selectively removing hydrogen sulfide from fluid streamscontaining hydrogen sulfide without producing a treated fluid streamcontaining significant amounts of sulfur dioxide. It is a further objectof this invention to provide an improved absorbing composition whichpossesses the property of being regenerable to the original absorbingcomposition state in the presence of oxygen when fully sulfided.

It has been found, in accordance with the present invention, that theaddition of silica to zinc oxide provides an absorbing composition thatis very effective in the removal of hydrogen sulfide from a fluid streamcontaining hydrogen sulfide, while significantly reducing, in comparisonto various known absorbing compositions, the amount of hydrogen sulfidethat is oxidized to sulfur dioxide during the absorption process .

DETAILED DESCRIPTION OF THE INVENTION

Thus, in accordance with the present invention, an absorbing compositionconsisting essentially of zinc oxide and silica is utilized toselectively remove hydrogen sulfide from a fluid stream containinghydrogen sulfide. Additionally, the absorbing composition may containone or more promoters, such as nickel oxide, and one or more binders.Once the absorbing composition of the present invention has beenprepared, fluid streams containing hydrogen sulfide are contacted withthe absorbing composition under suitable absorbing conditions tosubstantially reduce the concentration of hydrogen sulfide in the fluidstream without significantly increasing the concentration of sulfurdioxide therein.

It is believed that the hydrogen sulfide is being absorbed by theabsorbing composition and thus the terms "absorption process" and"absorbing composition" are utilized for the sake of convenience.However, the exact chemical phenomenon occurring is not the inventivefeature of the process of the present invention and the use of the term"absorb" in any form is not intended to limit the present invention.

The selective absorption process is preferably carried out in cyclescomprising an absorption period and a period for the regeneration of thesulfided absorbing composition. The absorption period comprisescontacting a gaseous stream which contains hydrogen sulfide with theabsorbing composition to thereby selectively remove hydrogen sulfidefrom the gaseous stream. The absorbing composition becomes sulfidedduring the absorption period. When the absorbing composition becomessulfided to the point that regeneration is desirable, preferably when itis nearly completely sulfided, an oxygen-containing gas is passed incontact with the absorbing composition to regenerate the absorbingcomposition and to convert the absorbed sulfur to a sulfur oxide.

The chemical changes that are believed to occur in the absorbingcomposition during this cyclic process are summarized in the followingequations:

    ZnO+H.sub.2 S→ZnS+H.sub.2 O                         (I)

    ZnS+Oxygen→ZnO+SO.sub.x                             (II)

Other objects and advantages of the invention will be apparent from theforegoing description of the invention and the appended claims as wellas from the detailed description of the invention which follows.

The absorbing composition of the present invention may be utilized toremove hydrogen sulfide from any suitable gaseous stream. The hydrogensulfide may be produced by the hydrodesulfurization of organic sulfurcompounds or may be originally present in the gaseous stream as hydrogensulfide. Examples of such suitable gaseous streams include lighthydrocarbons such as methane, ethane and natural gas; gases derived frompetroleum products and products from extraction and/or liquefaction ofcoal and lignite; gases derived from tar sands and shale oil; coalderived synthesis gas; gases such as hydrogen and nitrogen; gaseousoxides of carbon; steam and the inert gases such as helium and argon.Gases that adversely affect the removal of hydrogen sulfide and whichshould be absent from the gaseous streams being processed are oxidizingagents, examples of which include air, molecular oxygen, the halogens,and the oxides of nitrogen.

The absorbing composition of the present invention may be utilized toremove hydrogen sulfide from olefins such as ethylene. This process,however, should be carried out in the absence of free hydrogen to avoidhydrogenation. Olefin streams should not be hydrodesulfurized as thismay result in undesirable hydrogenation of at least a portion of theolefins to paraffins.

In one embodiment of the present invention, the absorbing compositionconsists essentially of zinc oxide and silica. In a second embodiment ofthe present invention, the absorbing composition consists essentially ofzinc oxide, silica, and nickel oxide. In a third, and preferred,embodiment of the present invention, the absorbing composition compriseszinc oxide and diatomite, preferably promoted with nickel oxide.Additionally, each of these absorbing compositions can further containbinders.

The absorbing composition employed in the process of the presentinvention may be prepared by any suitable method known in the art.Examples of such suitable methods include coprecipitation, dry mixing ofsolids, and slurrying. Once the absorbing composition has been prepared,it may be formed into a suitable contact material by any suitable methodknown in the art. Examples of such suitable methods include extrusion,pelletization, tabletting, and spray drying.

A preferred method for preparing the absorbing composition employed inthe process of the present invention is to combine silica, zinc oxide,and, if present, a binder in a mixer. After mixing these components toform a mixture thereof, a dilute acid is then added with continuedmixing to the resulting mixture to thereby form an extrudable paste. Theextrudable paste is then extruded, dried, and calcined to form theabsorbing composition. Due to the abrasive nature of silica, and itsdestructive effect on extrusion equipment, it may be desirable toinclude a lubricant, such as graphite, as a component in the originalmixture. The lubricant is then removed from the resulting compositionduring the calcination step, either by combustion or by conversion ofthe lubricant to a metal oxide promoter in accordance with the presentinvention.

In view of the equipment wear caused by extruding the abrasive silica,an alternative method for forming the absorbing composition may bedesired. One example of such a method is to pelletize a mixture ofsilica, zinc oxide, and, if present, a binder in the presence of adilute acid by employing a disk or drum pelletizer. The mixture ofsilica, zinc oxide, and, if present, a binder may be prepared in themanner described above, or may be prepared in other suitable apparatus,such as a double cone blender.

In a preferred embodiment of the present invention, nickel oxide or itsprecursor is added to the extruded, dried, and calcined composition, andthe now-promoted composition is dried and calcined a second time to formthe promoted absorbing composition. Alternatively, the nickel oxide orits precursor may be included as a component of the original mixture,thus reducing the number of times the composition must be dried andcalcined to form the promoted absorbing composition.

The zinc oxide used in the preparation of the absorbing composition maybe either in the form of zinc oxide, or in the form of one or more zinccompounds that are convertible to zinc oxide under the conditions ofpreparation described herein. Examples of such zinc compounds includezinc sulfide, zinc sulfate, zinc hydroxide, zinc carbonate, zincacetate, and zinc nitrate. Preferably, the zinc oxide is in the form ofpowdered zinc oxide.

The silica used in the preparation of the absorbing composition may beeither in the form of silica, or in the form of one or more siliconcompounds that are convertible to silica under the conditions ofpreparation described herein. Any suitable type of silica may be used inthe absorbing composition employed in the process of the presentinvention. Examples of suitable types of silica include diatomite,silicalite, silica colloid, flame-hydrolyzed silica, hydrolyzed silica,and precipitated silica, with diatomite being presently preferred.Examples of silicon compounds that are convertible to silica under theconditions of preparation described herein include silicic acid, sodiumsilicate, and ammonium silicate. Preferably, the silica is in the formof diatomite.

In accordance with the preferred method previously described herein,silica, zinc oxide, and, if present, a binder are initially combined ina mixer. To achieve the desired dispersion of these materials, thematerials are blended until a homogenous mixture is formed. The mixingtime will generally be in the range of about 1.0 minute to about 45minutes, and will preferably in the range of about 2.0 minutes to about15 minutes.

Any suitable binder may be added to the absorbing composition employedin the process of the present invention. A suitable binder is consideredto be any material that improves the physical strength of the finalabsorbing composition without having a significant adverse effect on theperformance of the absorbing composition in the process of the presentinvention, such as a significant increase in the amount of hydrogensulfide being oxidized to sulfur dioxide by the absorbing composition.Examples of suitable binders include alumina and calcium aluminate. Thebinders may be added in any form suitable for combination with thesilica and zinc oxide. For example, the binder may be in the form of asolid material, a dry powder, or a sol.

When the silica, zinc oxide, and, when present, binder have been blendedwithin the mixer for the desired amount of time, a dilute acid is thenadded, with continued mixing, to the resulting mixture to thereby forman extrudable paste. The dilute acid may be added to the resultingmixture by any suitable method. Preferably, the dilute acid is added tothe resulting mixture by spraying it within the mixer during continuedmixing.

Any suitable acid may be used in the preparation of the absorbingcomposition. Examples of suitable acids include nitric acid, aceticacid, sulfuric acid, and hydrochloric acid, with acetic acid beingpresently preferred. The acid concentration in the dilute acid employedin the preparation of the absorbing composition will generally be in therange of about 1.0 weight-% to about 15 weight-%, and will preferably bein the range of about 1.0 weight-% to about 5.0 weight-%, said weight-%being expressed in terms of the concentrated acid based upon the totalweight of the dilute acid.

As part of this invention, it has been discovered that the amount ofwater that is added to the resulting mixture, in the form of the diluteacid, to form the extrudable paste has an effect on the particle crushstrength of the resulting absorbing composition. Thus, to prepareabsorbing compositions having higher particle crush strengths, theamount of water added to the resulting mixture will generally be in therange of about 0.26 lbs water/lbs solids to about 0.38 lbs water/lbssolids, and will preferably be less than about 0.30 lbs water/lbssolids.

After adding the dilute acid to the resulting mixture, the acidizedmixture will generally continue to be mixed for a period of time in therange of about 5 minutes to about 60 minutes, preferably in the range ofabout 10 minutes to about 30 minutes, thereby forming the extrudablepaste.

The extrudable paste may either be directly extruded or may be agedprior to being extruded. As part of the present invention it has beendiscovered that whether or not the extrudable paste is aged has aneffect on the physical properties of the resulting absorbing compositionand, further, that this effect varies with the type and concentration ofdilute acid used and the liquids to solids ratio used in the preparationof the extrudable paste. Thus, for example, when employing acetic acidin the preparation of the absorbing composition, a higher crush strengthmaterial may be obtained by extruding the extrudable paste as soon aspractically possible, without significant aging. When low concentrationnitric acid is employed at a low liquids to solids ratio, however, ahigher crush strength material may be obtained by allowing theextrudable paste to age for a period of time prior to extruding it. Thedesirability of aging the extrudable paste, with respect to any suitableacid and liquids to solids ratio, may be determined by routineexperimentation. Thus, in accordance with the present invention, theextrudable paste may be aged for any suitable amount of time. Theextrudable paste will generally be aged for a period of time in therange of about 0 minutes to about 120 minutes, and will preferably beaged for a period of time in the range of about 0 minutes to about 60minutes.

The extrudable paste is then extruded by methods well known in the art.The extruded material is then dried at a temperature generally in therange of about 75° C. to about 300° C., and more preferably in the rangeof about 90° C. to about 250° C., for a period of time generally in therange of about 0.5 hour to about 4 hours, and more preferably in therange of about 1 hour to about 3 hours. The dried, extruded material isthen calcined in the presence of oxygen at a temperature generally inthe range of about 375° C. to about 750° C., and more preferably in therange of about 500° C. to about 700° C., for a period of time generallyin the range of about 0.5 hour to about 4 hours, and more preferably inthe range of about 1 hour to about 3 hours to produce the absorbingcomposition employed in the process of the present invention.

The zinc oxide will generally be present in the absorbing composition inan amount in the range of about 10 weight-% to about 90 weight-%, andwill more preferably be in the range of about 45 weight-% to about 90weight-%, and will most preferably be in the range of about 45 weight-%to about 60 weight-%, when said weight-%'s are expressed in terms of thezinc oxide based upon the total weight of the absorbing composition.

The silica will generally be present in the absorbing composition in anamount in the range of about 10 weight-% to about 90 weight-%, and willmore preferably be in the range of about 30 weight-% to about 60weight-%, when said weight-%'s are expressed in terms of the silicabased upon the total weight of the absorbing composition.

The binder, when present, will generally be present in the absorbingcomposition in an amount in the range of about 5.0 weight-% to about 30weight-%, and will more preferably be in the range of about 5.0 weight-%to about 15 weight-%, when said weight-%'s are expressed in terms of theweight of the binder compared with the total weight of the absorbingcomposition.

The absorbing composition employed in the process of the presentinvention may be promoted with suitable metal oxides. Examples ofsuitable metal oxides include the oxides of molybdenum, tungsten, one ormore metals selected from Group VIII of the Periodic Table, and anyother metal that is known to have hydrogenation ability of the typenecessary to reduce sulfur oxide species to hydrogen sulfide. In apreferred embodiment of the present invention, the absorbing compositionis promoted with nickel oxide.

The metal oxide promoter may be added to the absorbing composition inthe form of the elemental metal, metal oxide, and/or metal-containingcompounds that are convertible to metal oxides under the calciningconditions described herein. Some examples of such metal-containingcompounds include metal acetates, metal carbonates, metal nitrates,metal sulfates, metal thiocyanates, and mixtures of two or more thereof.

The elemental metal, metal oxide, and/or metal-containing compounds canbe added to the absorbing composition by any method known in the art.One such method is the impregnation of the absorbing composition with asolution, either aqueous or organic, that contains the elemental metal,metal oxide, and/or metal-containing compounds. After the elementalmetal, metal oxide, and/or metal-containing compounds have been added tothe absorbing composition, the promoted composition is dried andcalcined, as described hereinafter.

As previously noted herein, the elemental metal, metal oxide, and/ormetal-containing compounds can be added to the absorbing composition ascomponents of the original mixture, or they may be added after theabsorbing composition has initially been dried and calcined. If themetal oxide promoter is added to the absorbing composition after it hasinitially been dried and calcined, then the now-promoted composition isdried and calcined a second time to form the promoted absorbingcomposition. The now-promoted composition is generally dried at atemperature in the range of about 75° C. to about 300° C., morepreferably in the range of about 90° C. to about 250° C., for a periodof time generally in the range of about 0.5 hour to about 8 hours, morepreferably in the range of about 3 hours to about 5 hours. The dried,promoted composition is then calcined in the presence of oxygengenerally at a temperature in the range of about 375° C. to about 750°C., and more preferably in the range of about 500° C. to about 700° C.,until volatile matter is removed and the elemental nickel and/or thenickel-containing compounds are substantially converted to nickeloxides. The time required for this calcining step will generally be inthe range of about 0.5 hour to about 4 hours, and will preferably be inthe range of about 1 hour to about 3 hours.

The metal oxide promoter will generally be present in the absorbingcomposition in an amount ranging from about 0.1 weight-% to about 15weight-%, and will more preferably be in the range of about 2.0 weight-%to about 7.5 weight-%, most preferably about 6.0 weight-%, saidweight-%'s being expressed in terms of the metal oxide based upon thetotal weight of the absorbing composition.

The processes of the present invention can be carried out by means ofany apparatus whereby there is achieved an alternate contact of theabsorbing composition with the gaseous feed stream and, thereafter, ofthe absorbing composition with an oxygen-containing gas which isutilized to regenerate the absorbing composition. The process is in noway limited to the use of a particular apparatus. The process of thisinvention can be carried out using a fixed bed of absorbing composition,a fluidized bed of absorbing composition, or a moving bed of absorbingcomposition. Presently preferred is a fixed bed of absorbingcomposition.

In order to avoid any casual mixing of the gaseous feed streamcontaining hydrogen sulfide with the oxygen-containing gas utilized inthe regeneration step, provision is preferably made for terminating theflow of the gaseous feed stream to the reactor and subsequentlyinjecting an inert purging fluid such as nitrogen, carbon dioxide orsteam. Any suitable purge time can be utilized but the purge should becontinued until all hydrocarbon and/or hydrogen sulfide are removed. Anysuitable flow rate of the purge fluid may be utilized. Presentlypreferred is a purge fluid flow rate in the range of about 800 GHSV toabout 1200 GHSV.

Any suitable temperature for the processes of the present invention maybe utilized. The temperature will generally be in the range of about150° C. to about 600° C. and will more preferably be in the range ofabout 200° C. to about 450° C.

Any suitable temperature may be utilized to regenerate the absorbingcomposition from its sulfided form back to the original absorbingcomposition form. The temperature will generally be in the range ofabout 370° C. to about 815° C. As a result of parallel work, however, ithas been discovered that the higher temperatures required to initiatethe regeneration of ZnS to ZnO (i.e. about 650° C. and higher) has anadverse effect on the amount of sulfur dioxide that is produced duringthe subsequent absorption cycle. Due to the fact that the regenerationof NiS to NiO is an exothermic reaction, and the fact that this reactionis initiated at a lower temperature (i e about 425° C.) the presence ofnickel oxide in the absorbing composition employed in the process of thepresent invention allows the regeneration to occur at a lowertemperature, thereby preventing the adverse effect described above.Thus, the regeneration temperature is preferably in the range of about425° C. to about 600° C., most preferably about 425° C., to effect theregeneration within a reasonable time while not adversely affecting theproduction of sulfur dioxide in the treated gaseous feed stream.

Any suitable pressure can be utilized for the processes of the presentinvention. The pressure of the gaseous feed stream being treated is notbelieved to have an important effect on the absorption process of thepresent invention, and will generally be in the range of from aboutatmospheric to about 2,000 psig during the treatment.

Any suitable residence time for the gaseous feed stream in the presenceof the absorbing composition of the present invention can be utilized.The residence time expressed as volumes of gas at standard temperatureand pressure per volume of absorbing composition per hour will generallybe in the range of about 10 to about 10,000 and will more preferably bein the range of about 250 to about 2500.

When the absorbing composition is completely sulfided it will no longercombine with the hydrogen sulfide in the manner set forth in equation(I). When this condition occurs, hydrogen sulfide will begin to appearin the effluent flowing from the reaction and this will be an indicationthat the absorbing composition should preferably be regenerated. Thetime required for the absorbing composition to become completelysulfided will generally be a function of the concentration of sulfur inthe feedstock and feed rate employed.

When the absorbing composition becomes substantially completelysulfided, the absorbing composition is typically regenerated byterminating the flow of feed to the reactor and purging with an inertfluid such as nitrogen to remove any combustibles. A freeoxygen-containing gas is then introduced to the reactor for the purposeof oxidizing the zinc sulfide in accordance with equation (II).

The amount of oxygen supplied to the reactor during the regenerationstep will generally be sufficient to at least substantially removesulfur from the absorbing composition. The regeneration step isgenerally conducted at about atmospheric pressure. The temperature forthe regeneration step is generally maintained in the range of about 370°C. to about 815° C., and is more preferably maintained at about 425° C.in order to oxidize the zinc sulfide within a reasonable time.

The following examples are presented in further illustration of theinvention.

EXAMPLE I

In this example the experimental procedure for the removal of hydrogensulfide from gas streams containing hydrogen sulfide by means of varioussolid sorbent materials is described.

The tests were carried out in a single reactor unit comprising a 20 mmO.D. Quartz reactor and a 2 mm thermocouple well. The reactor, which wasmaintained at a pressure of about 1.7 psig, was operated in a fixed beddown flow mode using 10 grams of sorbent. Within the reactor, thesorbent was heated to the reaction temperature in a stream of nitrogen.When the desired temperature was attained, the nitrogen flow wasstopped, and the simulated sulfur plant gas and water vapor flows (thewater content was about 12% of the gaseous stream) were started. Thewater vapor was generated by pumping water through a heated line withinthe reactor. The reaction was carried out at a reaction temperature ofabout 425° C. and a gas hourly space velocity of 2500 cc/cccatalyst/hour. The composition of the simulated sulfur plant gas was asfollows: 4.35 volume-% hydrogen sulfide, 39.9 volume-% carbon dioxide,and 55.75 volume-% nitrogen.

The progress of the absorption was followed by measuring theconcentration of hydrogen sulfide and/or the sulfur dioxide in thereactor effluent after the water vapor had been condensed and removedfrom the effluent. The concentration of hydrogen sulfide and/or sulfurdioxide was measured with Draeger tubes that were suited to theconcentration ranges encountered.

Once the sorbents became fully sulfided, as evidenced byhydrogen-sulfide breakthrough, the flow of the simulated sulfur plantgas to the reactor was halted and the reactor was purged with nitrogenfor a period of about 20 minutes while being heated to a regenerationtemperature in the range of about 621° C. to about 676° C. The sulfidedsorbent was then regenerated in the presence of air for about 1.5 hours.Following regeneration, the reactor was again purged with nitrogen forabout 40 minutes while being cooled back down to the reactiontemperature of about 425° C. The nitrogen purge was then halted and thesimulated sulfur plant gas was fed to tile reactor to begin anotherabsorption cycle.

EXAMPLE II

This example describes the sorbent materials which were tested inaccordance with the procedures set forth in Example I.

Sorbent A: comprised Ni/ZnO/Al₂ O₃ with 7.0 weight-% Ni (as NiO), 46.5weight-% ZnO and 46.5 weight-% Al₂ O₂. Sorbent A was prepared in thefollowing manner: First, ZnO powder (Lot 052579; Alfa Products Division,Morton Thiokol, Inc.; Danvers, Mass.) was ground to a particle size of-200 mesh. Next, about 61.2 grams of α-alumina monohydrate weredispersed in 500 mL of water with stirring. 4.4 mL of concentratednitric acid were then added to the solution to form an acidic solutioncomprising alumina. Next, a ZnO hydrosol was prepared by slurrying 50.5grams of the ground ZnO powder in 150 ml of water. After stirring theacidic solution comprising alumina for about 10 minutes, the ZnOhydrosol was added, with rapid stirring, to the acidic solutioncomprising alumina, and a hydrogel of zinc oxide and alumina was quicklyformed. The hydrogel of zinc oxide and alumina was then transferred toan evaporating dish and dried at a temperature of about 120° C. forabout 12 hours. The dried hydrogel was then calcined in air at 500° C.for a period of 3 hours. The BET/N₂ surface area of the calcinedhydrogel was about 60 m² /g. 50 grams of the calcined hydrogel were thenimpregnated with a solution containing 17.3 grams of Ni(NO₃)₂. 6H₂ O(Lot 022381; Alfa Products Division, Morton Thiokol, Inc.; Danvers,Mass.) and 33 grams of H₂ O. Not all of the solution could be added tothe calcined hydrogel, so the partially-impregnated hydrogel waspartially dried under a heat lamp before continuing with theimpregnation of the rest of the solution. The impregnated hydrogel wasthen dried for 3 hours at about 110° C., and then calcined at 500° C.for an additional 3 hours to form Sorbent A.

Sorbent B: comprised Ni/ZnO/SiO₂ with 6.0 weight-% Ni (as NiO), 47weight-% ZnO and 47 weight-% SiO₂. Sorbent B was prepared in thefollowing manner: 50 grams of ZnO (St. Joe Minerals Corporation (nowHorsehead Industries); Palmerton, Pa.) were combined with 50 grams ofSiO₂ (Cab-o-sil silica; Cabot Corporation; Tuscola, Ill.). The combinedZnO and SiO₂ were dry mixed for about 3 minutes prior to being mulled ina solution containing 118.4 grams of H₂ and 1.48 grams of HNO₃, althoughonly 66.30 grams of the solution were used to reach a desiredconsistency. The ,resulting extrudate was dried overnight in air andthen dried at 110° C. for about 3 hours. The dried extrudate was thencalcined at about 500° C. for about 3 hours. About 25 grams of thecalcined extrudate was then impregnated with a solution containing 7.43grams of Ni(NO₃)₂. 6H₂ O (Lot 022381; Alfa Products Division, MortonThiokol, Inc.; Danvers, Mass.) and 18.25 grams of H₂. Afterimpregnation, the impregnated extrudate was dried under a heat lamp forabout 1 hour, then dried at about 110° C. for about 3 hours, and finallycalcined at 500° C. for 3 hours to form Sorbent B.

Sorbent C: comprised Ni/ZnO/SiO₂ with 6.0 weight-% Ni (as NiO), 47weight-% ZnO and 47 weight-% SiO₂. Sorbent C was prepared in thefollowing manner: 50 grams of ZnO (St. Joe Minerals Corporation (nowHorsehead Industries); Palmerton, PA) were combined with 50 grams ofSiO₂ (Diatomaceous earth; Fisher Scientific Company; Pittsburgh, Pass.).The combined ZnO and SiO₂ were dry mixed for about 3 minutes prior tobeing mulled in a solution comprising 118.44 grams of H₂ O and 1.48grams of HNO₃, although only 54.41 grams of the solution were used toreach a desired consistency. The resulting extrudate was dried overnightin air and then dried at 110° C for about 3 hours. The dried extrudatewas then calcined at about 500° C. for about 3 hours. About 25 grams ofthe calcined extrudate was then impregnated with a solution containing7.43 grams of Ni(NO₃)₂. 6H₂ O (Lot 022381; Alfa Products Division,Morton Thiokol, Inc.; Danvers, Mass.) and 9.32 grams of H₂ O. Afterimpregnation, the impregnated extrudate was dried under a heat lamp forabout 1 hour, then dried at about 110° C. for about 3 hours, and finallycalcined at 500° C. for 3 hours to form Sorbent C.

EXAMPLE III

This example illustrates the use of the sorbents described in Example IIwithin the procedure described in Example I for the removal of H₂ S froma simulated sulfur plant gas. The results are presented as a function ofthe amount of sulfur dioxide (measured in ppm) present in the effluentgaseous stream at a point in time 10 minutes into the absorption cycle,and of the total amount of sulfur absorbed by the sorbent (measured on aweight basis) at the time of hydrogen sulfide breakthrough. The cyclenumber listed is the number of the absorption cycle in which the readingwas taken during an ongoing test comprising repeated cycles ofabsorption and regeneration. The test results are summarized in Table I.

                  TABLE I                                                         ______________________________________                                                                    Cycle SO2   Sulfur                                Run    Sorbent  Composition #     Level Loading                               ______________________________________                                        1 (Con-                                                                              A        Ni/ZnO/Al.sub.2 O.sub.3                                                                    6    1800  9.5                                   trol)                        7    1680  8.5                                   2 (Inven-                                                                            B        Ni/ZnO/SiO.sub.2                                                                           1     0    10.0                                  tion)           (Cab-O-Sil)  5    600   9.9                                                                6    600   9.9                                                               12    600   9.9                                                               36    800   9.9                                   3 (Inven-                                                                            C        Ni/ZnO/SiO.sub.2                                                                           5    --    9.8                                   tion)           (Diatomite) 29    --    10.0                                                              34    --    9.0                                                               39    384   10.6                                                              45    352   10.7                                  ______________________________________                                    

A comparison of the results set forth in Table 1 clearly shows thatreplacing the alumina in Sorbent A with silica (Sorbents B and C) inaccordance with the present invention dramatically reduces the amount ofhydrogen sulfide that is oxidized to sulfur dioxide during theabsorption cycle of the inventive process, thereby significantlyreducing the amount of sulfur dioxide that passes unabsorbed through theabsorbing bed. Additionally, a review of the sulfur loadings set forthin Table 1 demonstrates that the absorbing compositions employed in theprocess of the present invention are highly effective in the removal ofhydrogen sulfide. Finally, a comparison of the results of Run 3 withthose of Runs 1 and 2 shows why diatomite is the presently preferredsilica for use in the absorbing composition employed in the process ofthe present invention.

EXAMPLE IV

This example_(i) describes the methods that were used to prepare thesorbent materials that are intended to illustrate the different methodsthat may be used in accordance with the present invention to add nickeloxide to the absorbing compositions of the present invention.

All of the sorbents prepared in this example, except for Sorbent G,comprise Ni/ZnO/SiO₂ /Al₂ O₃ with 6.0 weight-% Ni (as NiO), 47 weight-%ZnO, 38 weight-% Si₂, and 9.0 weight-% Al₂ O₃. The ZnO used to formthese sorbents was obtained from St. Joe Minerals Corporation, thesilica was Celite Filter-Cel Silica (a diatomite from Johns-ManvilleCorporation; Denver, CO), and the alumina was Catapal D (Vista ChemicalCompany; Houston, Tex.).

Sorbent D: Sorbent D was prepared by initially forming a nitric acidsolution which contains 80 mL of a nickel nitrate solution, Ni(NO₃)₂.6H₂ O mixed in water at a concentration of 1.073 g of nickel nitratehexahydrate/mL solution, and 35 mL of water. This solution was thenstirred for 10 minutes. Next, 113 grams of silica, 36 grams of alumina,142 grams of zinc oxide, and 3 grams of graphite are combined in a sigmamixer, and the resulting mixture is allowed to mix for about 5 minutes.The nickel nitrate solution is then added to the resulting mixture andmixing is continued for about 15 minutes. The resulting paste is thenextruded, and the extrudate is dried at 140° C. for about 3 hours. Thedried extrudate is then calcined at 635° C. for about 3 hours to formSorbent D.

Sorbent E: Sorbent E was prepared by initially combining, with stirring,15 mL of concentrated acetic acid and 112 mL of water to form an aceticacid solution. This solution was then stirred for about 10 minutes.Next, 113 grams of silica, 36 grams of alumina, 142 grams of zinc oxide,3 grams of graphite, and 22 grams of nickel oxide (Novamet SpecialtyProducts; Wyckoff, N.J.) are combined in a sigma mixer, and theresulting mixture is allowed to mix for about 5 minutes. The acetic acidsolution is then added to the resulting mixture, and mixing is continuedfor about 15 minutes. The resulting paste is then extruded, and theextrudate is dried at 140° C. for about 3 hours. The dried extrudate isthen calcined at 635° C. for about 3 hours to form Sorbent E.

Sorbent F: Sorbent F was prepared by initially combining, with stirring,0.316 lbs of alumina, 0.072 lbs of concentrated acetic acid, and 1.083lbs of distilled, water to form an alumina sol. 1.0 1 lb of silica wasthen added to the alumina sol and the resulting mixture was mixed forabout 10 minutes. 1.25 lbs of zinc oxide was then added to the resultingmixture and mixing was continued for about 10 minutes to form anextrudable paste. The resulting extrudable paste was allowed to age forabout 30 minutes and was then extruded to form an extrudate. Theextrudate was then dried at 140° C. for about 3 hours, and wassubsequently calcined at 635° C. for about 45 minutes. The calcinedextrudate was then spray impregnated with about 260 grams of a nickelnitrate solution (0.87 grams Ni(NO₃)₂. 6H₂ / mL solution) and 12.7 gramsof distilled water. The impregnated extrudate was then dried overnightat about at 140° C. The dried extrudate is then calcined at 635° C. forabout 45 minutes to form Sorbent F.

Sorbent G: Sorbent G comprises 6.0 weight-% Ni (as NiO), 47 weight-%ZnO, and 47 weight-% alumina. Sorbent G was prepared by initiallycombining 22.1 lbs of ZnO and 27.9 lbs of alumina. This mixture wasmixed for about 5 minutes. About 20.4 lbs of a 13.45 wt-% nitric acidsolution was then added to the mixture, and mixing was continued forabout 5 minutes to form an extrudable paste. The extrudable paste wasallowed to age for about 30 minutes, and was then extruded to form anextrudate. The extrudate was dried at 135° C. for about 3 hours, and wasthen calcined for about 45 minutes at 635° C. The calcined extrudate wasthen impregnated with a nickel nitrate solution, and the impregnatedextrudate was again dried and calcined, in the manner discussed above,to form Sorbent G.

EXAMPLE V

This example illustrates the use of the sorbents described in Example IVwithin the procedure described in Example I for the removal of H₂ S froma simulated sulfur plant gas. The results are presented as a function ofthe amount of sulfur dioxide (measured in ppm) present in the effluentgaseous stream at a point in time 10 minutes into the absorption cycle,and of the total amount of sulfur absorbed by the sorbent (measured on aweight basis) at the time of hydrogen sulfide breakthrough. The testresults are summarized in Table II.

                  TABLE II                                                        ______________________________________                                                                             Sulfur                                   Run       Sorbent  Cycle #   SO2 Level                                                                             Loading                                  ______________________________________                                        1 (Invention)                                                                           D         1        --      12.3                                                         2        548     13.4                                                        14        545     10.8                                                        19        525     13.4                                                        23        541     14.0                                                        24        514     13.7                                                        27        563     13.8                                                        32        528     13.5                                                        35        528     14.1                                                        50        510     13.3                                     2 (Invention)                                                                           E         2        --      12.1                                                         6        553     13.4                                                        10        470     12.7                                                        23        487     13.0                                                        27        600     14.0                                                        40        600     13.8                                     3 (Invention)                                                                           F         1        --      13.3                                                         2        600     12.8                                                         6        600     12.4                                                         8        600     11.5                                                        11        600     12.2                                                        33        380     10.4                                                        39        371     10.7                                                        44        340     10.2                                                        50        346     10.2                                     4 (Control)                                                                             G        13        1500    6.1                                                         17        1000    7.3                                                         29        1100    7.4                                                         35        1200    7.5                                                         42        1100    7.9                                                         54        900     6.9                                      ______________________________________                                    

The data contained in Table II demonstrates that the nickel promoteremployed in one embodiment of the present invention can be included asan original component in the preparation of the absorbing composition(Sorbents A and B) or that it may be added as a promoter to an existingabsorbing composition (Sorbent C). Additionally, the data demonstratesthat the nickel promoter may be added in the form of nickel oxide(Sorbent B), or in the form of a nickel compound that is convertible tonickel oxide (Sorbent A). Regardless of how the nickel promoter is addedto the absorbing composition, it is evident from the above data that theresulting composition is an effective absorbing composition for hydrogensulfide, that does not readily oxidize hydrogen sulfide to sulfurdioxide during the absorption cycle of the process of the presentinvention. Finally, a comparison of Run 4 with Runs 1-3 clearlyillustrates the superiority, in terms of both an ability to absorbsulfur and avoid oxidizing hydrogen sulfide to sulfur dioxide, of theinventive absorbing compositions over an absorbing compositioncontaining alumina.

While this invention has been described in detail for purposes ofillustration, it is not to be construed as limited thereby but isintended to include all reasonable variations and modifications withinthe scope and spirit of the described invention and the appended claims.

That which is claimed is:
 1. An absorbing composition comprising zincoxide and silica in the substantial absence of alumina.
 2. A compositionin accordance with claim 1 wherein said zinc oxide is present in saidcomposition in an amount in the range of about 10 weight-% to about 90weight-%, said weight-% being expressed in terms of the zinc oxide basedupon the total weight of the composition.
 3. A composition in accordancewith claim 2 wherein said zinc oxide is present in said composition inan amount in the range of about 45 weight-% to about 90 weight-%.
 4. Acomposition in accordance with claim 2 wherein said zinc oxide presentin said composition in an amount in the range of about 45 weight-% toabout 60 weight-%.
 5. A method for preparing an absorbing compositioncomprising zinc oxide and silica in the substantial absence of aluminacomprising the steps of:a.) mixing zinc oxide or a precursor of zincoxide with silica or a precursor of silica to form a homogeneous mixturethereof; b.) adding a dilute acid to said homogeneous mixture to form anextrudable paste; c.) extruding said extrudable paste to form anextrudate; d.) drying said extrudate; and e.) calcining the driedextrudate to produce said composition.
 6. A method in accordance withclaim 5 wherein said precursor of zinc oxide is selected from the groupconsisting of zinc sulfide, zinc sulfate, zinc hydroxide, zinccarbonate, zinc acetate, and zinc nitrate; said precursor of silica isselected from the group consisting of silicic acid, sodium silicate, andammonium silicate; said dilute acid is a dilute solution of an acidselected from the group consisting of nitric acid, acetic acid, sulfuricacid, and hydrochloric acid; the concentration of said acid in saiddilute acid is in the range of about 1.0 weight-% to about 15 weight-%based upon the total weight of the dilute acid; the amount of diluteacid added to said homogeneous mixture results in an amount of waterbeing added to said homogeneous mixture in the range of about 0.26 lbswater/lbs solids to about 0.38 lbs water/lbs solids; said extrudate isdried at a temperature in the range of about 75° C. to about 300° C. fora period of time in the range of about 0.5 hour to about 4 hours; and,said dried extrudate is calcined in the presence of oxygen at atemperature in the range of about 375° C. to about 750° C. for a periodof time in the range of about 0.5 hour to about 4 hours to produce saidcomposition.
 7. A composition in accordance with claim 4 wherein saidcomposition is prepared by the process comprising the steps of:a.)mixing powdered zinc oxide with silica to form a homogeneous mixturethereof; b.) adding dilute acetic acid, having an acid concentration inthe range of about 1.0 weight-% to about 5.0 weight-% based upon thetotal weight of the dilute acid, to said homogeneous mixture to form anextrudable paste; c.) extruding said extrudable paste to form anextrudate; d.) drying said extrudate at a temperature in the range ofabout 90° C. to about 250° C. for a period of time in the range of about1 hour to about 3 hours; and, e.) calcining the dried extrudate in thepresence of oxygen at a temperature in the range of about 500° C. toabout 700° C. for a period of time in the range of about 1 hour to about3 hours to produce said composition.
 8. A composition in accordance withclaim 7 wherein the amount of dilute acetic acid added to saidhomogeneous mixture results in an amount of water being added to saidhomogeneous mixture in the range of about 0.26 lbs water/lbs solids toabout 0.30 lbs water/lbs solids.